The present invention relates to a method of shielding a printed circuit board, or a part of it, from disturbances caused by electromagnetic interference (EMI), the printed circuit board or its part being enclosed in a shielding housing which comprises a cover part with adjoining side parts. The invention also relates to a shielding housing for use in the method.
It is known that in electronic equipment in which the packing density, of components and the frequencies used are relatively high, there is the problem of electromagnetic coupling between different functional circuits. This coupling, i.e. electromagnetic interference (EMI), causes disturbances in the functioning of the circuits. The circuits have to be shielded in order to prevent this coupling. High requirements are set on the shielding methods with respect to shielding effect, the space requirement of the shield, its weight, price, usability for shielding parts of a printed circuit, and the detaching and attaching properties of the shield.
It is well known in the art to place the entire electric circuit to be shielded in a metal shielding housing, from which signals travel via interference-shielded connectors to other circuits. However, such a solution is space-consuming, increases the weight of the device, and is not very practical when there are several areas to be shielded separately on the same printed circuit board.
It is also known to shield certain parts on a printed circuit board with various metal shielding covers. In such a case, various methods are used. In such method a housing with a cover and side walls is placed on the printed circuit. At the lower edges of the side walls there are tabs which, during the installation of the housing, pass into apertures in the printed circuit board and are bent on the other side of the board into an orientation parallel to the lower surface of the board, thus securing the housing firmly to the board. For purposes of maintenance of the printed circuit, the bent tabs are straightened out, whereupon the housing can be detached. Such a method is described in, for example U.S. Pat. No. 4,370,515. Another method involves encasing the circuit to be shielded in a metal wall, the lower edge of which is soldered to the printed circuit board. A cover is placed so as to bear on the wall, the edges of the cover are flexibly bent so that, when the cover is pressed over the upper edge of the wall, the flexible cover edges will produce a locking effect. These methods are highly usable when the lower surface of the printed circuit board is a continuous ground foil which serves as a shield on that side. However, the circuit board material used must be relatively thick in order to maintain the elastic property and, if the lower surface of the printed circuit board is not a continuous metal foil, arranging shielding on this side is cumbersome, and thus it is difficult to achieve a perfect shielding effect.